A rate-responsive pacemaker adjusts its pacing rate in accordance with the value of a measured parameter, called a rate control parameter (RCP). The RCP varies with the metabolic needs of the body and the value of the RCP measurement depends upon whether a patient is under stress, exercising or at rest. A rate-responsive pacemaker generally incorporates some parameter which relates the desired pacing rate as a function of the RCP. This parameter is a rate response factor (RRF), also called a "slope", which is programmed to select the degree of change in pacing rate that will occur for a given change in a sensed metabolic demand parameter (for example, minute volume). To program the RRF, a physician programs upper and lower pacing rate limits, then has a patient perform an exercise test to correctly ascertain the RRF.
In one example of a minute volume-based rate-responsive pacemaker (e.g. the pacemaker disclosed in U.S. Pat. No. 4,901,725, entitled "Minute Volume Rate-Responsive Pacemaker", issued Feb. 20, 1990 to T. A. Nappholz et al.), the pacer is implanted and programmed into an adaptive mode, in which the minute volume is sensed and calculated but the pacing rate does not respond to changes in minute volume. The patient is instructed to rest for at least an hour prior to the exercise test. During this time, the rate responsive sensing circuit adapts to the patient's individual respiratory impedance characteristics. Following the rest period, with the pacemaker remaining in the adaptive mode, the patient performs a near-maximal exercise test. The pacemaker calculates minute volume measurements for the rest and peak exercise conditions and, taking into account the programmed maximum and minimum pacing heart rates, the pacemaker and programmer determine a suggested optimal RRF value, which is displayed by the programmer. This is a recommended RRF value which the physician may program into the pacemaker. Similarly, other rate responsive pacemakers, such as those which base their rate adaptation upon activity, QT-interval, respiratory rate, central venous temperature, right ventricular pressure and other sensor measurements, determine a pacing rate dependent upon a programmable slope that correlates the sensed measurement to pacing rate.
One of the disadvantages of these rate-responsive pacemakers, which employ a programmable RRF function, is that the desired relationship between the measured rate control parameter and pacing rate does not remain constant for the life of a pacemaker or even from one programming to the next. For some rate-responsive pacemakers the RCP sensor is attached to a pacemaker lead or the RCP measurement is derived from an electrocardiogram signal which is sensed from the lead or leads. In either case, a change in lead position may cause an upward or downward shift in measured RCP values, leading to chronically elevated or lowered pacing rates. Furthermore, sensed RCP values vary with changes in sensor sensitivity or administration of drugs.
Another disadvantage of pacemakers with programmable RRF functions is that, in most cases, RRF initialization and programming procedures are complex and inefficient.
U.S. Pat. No. 4,856,522, entitled "Rate-Responsive, Distributed-Rate Pacemaker", issued Aug. 15, 1989 to J. C. Hansen, describes a rate-responsive heart pacer which modifies a rate response factor over time by arranging measured RCP values in a percentile ranking and mapping them into a percentile ranking of a desired rate distribution. By monitoring the RCP values over an extended time interval and developing a corresponding percentile ranking, the pacemaker automatically self-adapts to long-term changes in RCP measurements and insures that the desired rate distribution is obtained. One disadvantage of the Hansen pacer is that the range of pacing rates is determined by programming, so that the relationship between a particular RCP measurement and a pacing rate is based upon a prediction of what a proper relationship might be, rather than on the metabolic needs of a patient. A second disadvantage of the Hansen pacer is that it assumes that a patient will perform activities which exhibit a whole range of RCP values and require a whole range of heart rates on nearly a daily basis. Furthermore, the Hansen pacer will not respond rapidly to sudden changes in RCP sensitivity, such as those that result from a change in lead position. In addition, the Hansen pacer requires a relatively large amount of data storage memory for an implantable device. This disadvantage will become less critical as circuit technology evolves.
U.S. Pat. No. 5,085,215, entitled "Metabolic Demand Driven Rate-Responsive Pacemaker", issued to T. A. Nappholz et al. on Feb. 4, 1992, describes a dual-chamber rate-responsive pacemaker which senses two indicators of metabolic demand, the natural sinus rate and minute volume. This pacemaker continuously performs analysis of these two sensed parameters to determine whether a DDD or VVI pacing mode is most appropriate at a given time, at what rate pacing should be performed and, if a DDD mode is most appropriate, what time delay between an atrial heart beat and a ventricular pace should be set.
The primary object of the present invention is to provide for an analysis of natural sinus (atrial) rate and minute volume for a function not heretofore performed, namely, to determine an appropriate rate response factor based on the true metabolic needs of the body, as indicated by the natural atrial rate, but only under circumstances in which the natural atrial rate is functioning in a reliable manner.
It is another object of the present invention to provide for smoothing of the pacing rate when the pacemaker switches operations from one mode to another.
Further objects and advantages of the present invention will become apparent as the following description proceeds.